The acronym xDSL stands for the family of Digital Subscriber Line technologies, which allow high-speed access to the Internet and multimedia services over the local loop, which connects the CP (customer premises) to the CO (central office), that is over simple twisted pair cables. An xDSL transceiver at the CO communicates with an xDSL transceiver at the CP over the local loop.
Since decades the local loop, which is a transmission line consisting of two twisted copper wires, also called unshielded twisted pair (UTP), has given the customer access to POTS (Plain Old Telephony Service). The POTS signal, transmitted over the local loop, is analog and contained in the frequency band up to 4 kHz, which corresponds to the spectral content of speech.
xDSL exploits the frequency band above 4 kHz up to several MHz, which is not used by POTS. However as the legacy local loops have been engineered for voice-band transmission, there are no guarantees about the quality of the local loop with respect to transmission in this higher frequency band. Consequently it is preferable if every loop is qualified for xDSL.
The quality of a loop is expressed by its theoretical channel capacity, which is equivalent to the upper bound of the achievable bit rate. The capacity depends on the signal-to-noise ratio (SNR) as a function of the frequency at the receiver at the CP, respectively the CO, for the downstream transmission (from the CO to the CP), respectively the upstream transmission (from the CP to the CO). The SNR at the receiver at the CP, respectively the CO, is determined as a function of the frequency by the transfer function of the loop between the CO and the CP and the noise PSD (Power Spectral Density) at the CP, respectively the CO given the PSD of the transmitted signal at the CO, respectively the CP.
                    C        =                              ∫            0            W                    ⁢                                                    log                2                            (                              1                +                                                      P                    ⁡                                          (                      f                      )                                                                            N                    ⁡                                          (                      f                      )                                                                                  ⁢                                                          )                        ⁢                          ⅆ              f                                                          (        1        )            P(f)=|H(f)2S(f)  (2)
In general the local loop consists of a network of transmission lines. Every line in the network is a UTP characterized by its length and type. The line type specifies the cross-sectional geometrical dimensions, such as the wire diameter (also called wire gauge), and the material physical constants, such as the electrical permittivity of the dielectric separating the 2 copper wires. The most used wire diameters are 0.4 mm, 0.5 mm, and 0.6 mm. Polyethylene (PE) is the most occurring insulator but other materials are also used such as paper and PVC.
The network topology of the local loop is limited to a tree structure. The simplest topology is a single line. The magnitude of the transfer function reflects the attenuation of the line, which gets worse with increasing frequency and line length. Another topology that exists for long loops is a cascade of 2 or more lines with increasing wire diameter from the CO to the CP. For this topology reflections are caused by the change of the wire diameter at the splices connecting 2 lines. A topology that is also frequently encountered, especially in the USA, is a loop with 1, 2 or more bridged taps. A bridged tap is an open-ended line spliced to the main line. Reflections appear for this topology at the splice connecting the bridged tap to the loop and at the open end of the bridged tap. Reflections have a negative impact on the transfer function, because they interfere with the signal propagating along the direct path. For those frequencies for which the interference is destructive, the magnitude of the transfer function reduces. Such reductions rarely appear in the voice band because the bridged taps are usually not too long.
To improve the voice-band transmission for very long loops it has been a common practice to install loading coils, which are inductors of typically 88 mH, inserted in series in the loop. Typically the first loading coil is at 900 m from the CO and subsequent loading coils are spaced 1800 m apart. These loading coils act as low-pass filters with 4 kHz as cut-off frequency, which has such a bad effect on the transfer function above 4 kHz that xDSL cannot operate.
As the twisted pairs constituting the local loop are unshielded, external electromagnetic waves may couple into the loop and propagate towards the CO and the CP causing noise at the receiver. The electromagnetic coupling is reduced by the twisting of the 2 wires, because adjacent segments of the twisted pair experience electromagnetic waves with opposite polarity. In addition the twisting improves the balance of the line. A line is balanced when the 2 conductors have an equal impedance towards the earth. The balancing of the line prevents a common-mode signal from transforming into a differential-mode signal. In the case of a common-mode signal the 2 wires carry equal currents and the return path of the current is the ground. For a differential-mode signal the 2 wires carry opposite currents (out of phase currents). Electromagnetic waves may couple into the line because of imperfect twisting, and the common-mode signal that they cause, may transform into a differential-mode signal because of imperfect balancing, which is correlated with the twisting. Balance decreases with increasing frequency.
The noise is divided into 2 different types according to the origin of the external electromagnetic waves coupling into the loop. The first type of noise is crosstalk, which is the electromagnetic coupling between twisted pairs in the same cable. The cables leaving the CO contain thousands of twisted pairs. The closer to the CP the less pairs there are present in a cable. A difference is made between near-end crosstalk (NEXT) and far-end crosstalk (FEXT). The transmitters at the CO, respectively CP, are the source of NEXT for the near-end receivers at the CO, respectively the CP, and are the source of FEXT for the far-end receivers at the CP, respectively the CO. In general crosstalk gets worse with increasing frequency.
A second type of noise is radio-frequency interference (RFI), which is caused by radio waves coupling into the local loop, that acts as an antenna, especially if there are aerial lines. There are 2 major sources of radio waves in the frequency band of xDSL namely AM radio and amateur radio.
Hence, the local loop has several impairments for transmission in the frequency band of xDSL, which are not present for voice-band transmission. The existing metallic line testing (MLT) systems for POTS are not capable of qualifying the loops for xDSL accurately. Telecom operators need a reliable qualification tool for the deployment of xDSL.
For example, if a customer requests a certain xDSL service, the operator has to be able to estimate the achievable bit rate for the loop that connects the CP of that customer to the CO, especially when a precisely specified quality of service (QoS) in terms of bit rate is to be guaranteed. If the achievable bit rate is underestimated, there is the risk of lost business. If the bit rate is overestimated, the telecom operator risks ending up with a dissatisfied customer and a useless installation as well as troubleshooting costs. As the latter risks are the most important, the telecom operator will subtract a safety margin from the estimated bit rate. In this way the risk of loosing customers and having non-refunded costs is reduced at the expense of increasing the risk of lost business. The more accurate the estimate of the bit rate is, the smaller the safety margin can be, and the less lost business. Hence there is a need for an estimation of the bit rate which should be as accurate as possible, but without incurring large additional costs.
Currently there are systems available on the market for xDSL loop testing, but they require measurements at the CO and at the CP. So a technician has to visit the CP as well as the CO. Furthermore, the interpretation of the measurement results, which requires a lot of expertise, is mainly left to the technician. This makes the loop qualification labour intensive and hence too expensive, in general. Telecom operators are not eager to use this type of qualification scheme on a large scale. The way the loop qualification is actually handled depends on the operator. If there is a database available containing information about the loops in the access network, this knowledge can be used for the loop qualification. If the topology and the lengths and types of the individual line sections of the loops are available, then the transfer functions of the loops can be computed. In order to determine the theoretical channel capacity, the noise PSD at the CO and at the CP is still missing, but the transfer function is already very informative. However not all telecom operators maintain such a database. Furthermore, if the database exists, it often contains a lot of errors, because it is difficult to keep it up-to-date. Without a database a telecom operator has to make do with rules of thumb. For example the distance from the CP to the CO can be used as very rough criterium for loop qualification. The conclusion is that telecom operators need a completely automated xDSL loop qualification system that performs measurements at the CO end of a loop and can interpret the results in order to estimate the achievable bit rate of the loop as accurately as possible. When the loop qualification has been finalized, this system can be used as an xDSL loop testing system for maintenance purposes.
In order to determine the achievable bit rate of a loop, the transfer function of the loop and the noise PSD at the CP, respectively the CO, are needed for the downstream, respectively upstream transmission. These quantities have to be measured from the CO end of the loop. For the noise PSD at the CO this is trivial. The loop transfer function and the noise PSD at the CP cannot be measured directly at the CO.
The quantity that is directly measurable at the CO, which is named port 1, is the scattering parameter S11, which is the ratio of the reflected wave and the incident wave at port 1. The reflected wave is also called a reflectogram and the measurement procedure is named reflectometry, more specifically time-domain reflectometry (TDR) if the waves are measured in the time domain. As the loop is considered as a 1-port, the scattering parameter S11 equals the reflection factor and the loop is completely characterized. However as the loop is designed for transmission between a CO and a CP, it has to be considered as 2-port with the CO as port 1 and the CP as port 2. Then the loop is completely identified by its 4 scattering parameters S11, S21, S12, and S22. The transfer function is related to the S21 scattering parameter. Time-domain reflectometry (TDR) is a well known measurement technique, which has been applied since a few decades in different domains such as cable fault location. Since the beginning of the xDSL deployment TDR has been regarded as a candidate technology to solve the loop qualification problem. There are now xDSL loop testing systems available with an integrated TDR meter, but the interpretation of the measured reflectogram is left to the technician. The loop qualification performed with these systems aims at the detection, location, and removal of local loop impairments such as bridged taps and load coils.
Recently attempts have been made to process the reflectogram in order to estimate the theoretical channel capacity of the loop. This processing is done by means of a artificial neural network (ANN), which transforms a number of inputs into a number of outputs by means of some elementary mathematical operations such as addition or multiplication with scalar, and/or non-linear functions. These operations are structured in a way that resembles a real neural network. The specific structure and the values of the multiplying scalars, called weighting factors, determine the input/output behaviour of the ANN.
Some features of the reflectogram such as the position and height of the peaks, corresponding to the reflected pulses, are used as inputs and some interesting parameters of the loop are used as outputs such as the attenuation of the loop at a number of frequencies.
There are a number of disadvantages related to the use of an ANN network. The ANN has to be trained, i.e. its structure has to be defined and the weighting factors have to be tuned, using a large number of sets of inputs and corresponding outputs. The training procedure has to cover all combinations of loop and noise types present in the network. Extrapolation with respect to the training set is unreliable. For example if the training has been done for loops up to 2 km, then the neural network will give an unpredictable output for a loop of 3 km. Only interpolation is allowed, but validation is necessary. Due to the increasing complexity of the ANN with an increasing number of input and output arguments, the number of inputs and outputs of the ANN has to be limited. This means that not all the information present in the reflectogram is used and this has a negative impact on the accuracy. In addition only a few parameters related to the loop can be predicted. An ANN is a black box model. It doesn't allow inclusion of some priori knowledge about the loop that is available, e.g. the fact that a loop consists of a network of transmission lines. This lowers the accuracy.